1. Technical Field
This disclosure relates to an image writing device that writes an image on a surface of a photoconductor moving in a sub-scanning direction at a predetermined speed by repeatedly exposing the surface of the photoconductor along a main-scanning direction with an exposure head having a plurality of light-emitting elements arranged in the main-scanning direction, an image forming apparatus equipped with the image writing device, and an image writing method.
2. Related Art
Electrophotographic image forming apparatuses are widely used as a copier, a printer, a facsimile machine, a digital multifunction peripheral, or the like. Such apparatuses are equipped with an image writing device that exposes a surface of a photoconductor to write an image, i.e., form an electrostatic latent image, thereon. The image forming apparatus develops the electrostatic latent image formed on the surface of the photoconductor by the image writing device with a developer such as a toner to form a toner image, transfers the toner image onto a recording medium such as a sheet, fixes the toner image on the recording medium, and outputs the recording medium to the outside of the image forming apparatus.
Although formerly a laser-writing (i.e., raster optical system) type of image writing device used to be the dominant type of image writing device included in the above-described image forming apparatus, an image writing device employing a fixed writing system with an exposure head like the above-described one has been increasingly used in recent years. A light-emitting diode (LED) array including a plurality of LED elements serving as light-emitting elements arranged in the main-scanning direction at a density corresponding to the resolution is typically used as the exposure head.
The image writing device with the LED array exposes the charged surface of the photoconductor to the light emitted by the LED elements of the LED array, to thereby write an image, i.e., form an electrostatic latent image, on the photoconductor. ON and OFF of the LED elements in the LED array are controlled by an LED array drive unit based on image data to be written, which is stored in a line memory for each main-scanning line and transmitted to the LED array drive unit at line periods each corresponding to the resolution.
In such an image writing device or an image forming apparatus equipped with such an image writing device, if a change occurs in the moving speed in the sub-scanning direction of a surface of an image bearer such as a photoconductor, image unevenness (i.e., density unevenness) and image misregistration occur in the sub-scanning direction. Herein, the image bearer such as a photoconductor corresponds to a photoconductor such as a photoconductor drum or a photoconductor belt or a member having a surface to which an image written on the surface of the photoconductor is directly or indirectly transferred, and which moves in the sub-scanning direction.
There are methods to correct such image unevenness and image misregistration due to the change in the moving speed in the sub-scanning direction of the surface of such an image bearer. For example, a change in the rotation speed of a photoconductor drum serving as the image bearer may be detected with an encoder, and the timing of generating a line synchronization signal (i.e., horizontal synchronization signal) HSYNC adjusted based on the detection result to correct the image unevenness and image misregistration in the sub-scanning direction.